Vertices with anything above 0% weight move as if they were 100% weight?
Yes, this is correct behaviour.

The weights associated with bones on a vertex are normalized before the deformation.

A vertex, which is not assigned to any bone vertex group will not move at all. A vertex with two (or more) groups will have its weights normalized. If GroupA and GroupB have weights of 0.4 and 1.6, BoneA and BoneB will the influence the vertex's position by 20% and 80%.

If (as in your case) a vertex only belongs to a single group, that group is "normalized" which results in 100%.